This extension comes with several APIs to help you troubleshoot problems. These can be run via /civicrm/api or via drush if you are using Drupal (drush cvapi Mjwpayment.XXX).
For supported payment processors (ie. those that use Mjwshared) you can retry IPN notifications using this API.
system_log_id: The stored notification log entry to retry. Find using SystemLog.get API.